1. Field of the Invention
This invention relates to the manufacture of ceramic substrates, such as multilayer ceramic (xe2x80x9cMLCIxe2x80x9d) substrates and, more particularly, to the process of manufacturing MLC substrates having input-output pad surface metallurgy with increased resistance to structural failure.
2. Description of Related Art
In order to satisfy the increasing need for higher performance packaging of integrated circuits, MLC substrates are being developed with higher density input-output (xe2x80x9cI/Oxe2x80x9d) pad structures. However, the use of new materials to build the required I/O pad structures as well as changes in the processing of ceramic substrates after sintering, has increased the incidence of substrate mechanical failures at the metal-ceramic interface, such as ceramic tear out (xe2x80x9cCTOxe2x80x9d) of the I/O pads.
Generally, a weak I/O pad structure is one which fails when a low pulling force is applied to the I/O pad during I/O pad strength testing. A particular case known as xe2x80x9clow forcexe2x80x9d structural failure is understood as failures of the I/O pad when a pull force of less than 10 pounds is applied in a pin-pull test. In a typical Alumina MLC package where, for example, 21-pound pins are used, the I/O pad is expected to withstand 21 pounds of force applied to the I/O pin in a pin-pull test without structural damage. If a force in excess of 21 pounds is applied to the I/O pin it is expected that shank failure of the I/O pin will occur. In this example, a weak I/O pad is one which structurally fails when a force of less than 21 pounds is applied to the I/O pin.
Certain of the mechanisms underlying the low force failures of I/O pad structures are known, and include pin-shank or solder ball failure, ceramic tear out, planar failures at the metal-ceramic interface and braze failure. In the case of high strength CTO, the I/O pin pulls out a large piece of the ceramic structure with it, the piece typically having a diameter larger than a third of the size of the I/O pad diameter. In Alumina substrates built with 21-pound pins, this type of failure occurs for a pin-pull force typically above 10 pounds and is directly dependent on the brazing material volume located in the proximity of the I/O pad perimeter. Lower amounts of brazing material near the I/O pad perimeter, increased pin centrality within the I/O pad, and optimized brazing material volume near the center of the I/O pad, typically minimizes this type of failure. A ceramic braze dam is sometimes used to control this type of problem. In the case of a low strength CTO pin-pull failure, the I/O pin pulls out a small piece of the ceramic, typically less than a third of the size of the I/O pad. On an Alumina substrate built with 21-pound pins, a low strength CTO pin-pull failure typically occurs for a pin-pull force less than 10 pounds. An extreme case of failure occurs when most of the fracture line travels along the metal-ceramic interface and little or none of the ceramic is pulled out with the I/O pin. This case, also classified as planar or interface failure, occurs less frequently. On an Alumina substrate built with 21-pound pins, a planar failure can easily occur for pin-pull force below 5 pounds.
While there is a general understanding of the mechanisms underlying these failures, an unacceptable fraction of the low force failures occur in Alumina MLC""s for unknown reasons. Methods to reduce the incidence of these low force metal-ceramic interface failures have included the use of a ceramic braze dam placed above the I/O pad after sintering to reduce the stress caused by the braze-pin structure at the edge of the I/O pad. However, the ceramic dam does not resolve the problem of the I/O pin failing at the center of the I/O pad.
It is also known to alter the composition and manufacture of ceramic green sheets to control the strength of the ceramic material. U.S. Pat. No. 5,045,402 discloses a method of producing a toughened glass ceramic through the use of Zirconia particles. While this process produces a green sheet capable of withstanding increased stresses on the substrate, it does not improve the metal-ceramic interface binding strength under an I/O pad.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method and structure to reduce the incidence of low strength metal-ceramic interface failure under an I/O pad on an MLC substrate.
A further object of the invention is to provide a method and structure to increase the pin-pull strength of an I/O pad on an MLC substrate.
It is yet another object of the present invention to provide a method and structure to eliminate the need for a ceramic braze dam over an I/O pad on an MLC substrate.
It is yet a further object of the invention to modify the MLC metal-ceramic topology to change ceramic tear out population distribution and reduce the frequency of low strength CTO.
Still another object of the invention is to reduce the incidence of pin-pull test failures below 10 pounds of applied force on MLC substrates.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of manufacturing a multilayer ceramic board having increased resistance at a metal-ceramic interface comprising the steps of:
(a) providing a green sheet having a surface on which an input-output pad is to be disposed;
(b) roughening a portion of the surface in those areas corresponding to where the input-output pad is to be disposed;
(c) screening the input-output pad over the roughened portion of the surface; and
(d) heat treating the green sheet and the input-output pad to create a bond between the pad and the surface.
In the preferred embodiment, the roughened portion of the surface extends to a depth of at least 1 mil below the surface of the green sheet.
The roughened portion of the surface of the green sheet may be formed by using a silicone coated paper or fibrous backing sheet in the casting of the green sheet or running the green sheet through a laminator to roughen the surface.
In another preferred embodiment the roughened surface of the green sheet is formed by providing a template onto which the roughness feature has been cast and embossing the surface roughness feature onto the green sheet.
In one preferred embodiment, the roughened surface is formed by depositing a concentric ring onto the green sheet over those areas where an input-output pad is to be disposed, or alternatively, screening the input-output pad over the roughened portion of the green sheet and then depositing the concentric ring over the input-output pad.
In the preferred embodiment, the shape of the ring corresponds to the shape of the input-output pad.
In yet another aspect, the roughened surface can be formed by providing holes on the green sheet in those areas where the input-output pad is to be disposed. In the preferred embodiment, the depth/size aspect ratio of the holes is less than 2. The holes on the green sheet may be formed by:
(i) providing a template having alumina particles extending from a surface;
(ii) contacting the green sheet with the template surface; and
(iii) applying pressure to the template in order to create holes in the green sheet in those locations corresponding to where the input-output pad is to be disposed.
The average size of the alumina particles on the template is preferably in the range of 20-100 microns.
In yet another embodiment, the roughened surface of the green sheet is formed by imprinting the green sheet with a patterned surface in those areas where the input-output pad is to be disposed. The pattern imprinted on the green sheet is preferably an array of three dimensional shapes such as squares, rectangles, pyramids, inverted cones, dots, holes, semicircles, circles and grooves.
In yet another aspect, the present invention provides a method of manufacturing a multilayer ceramic board having increased resistance to structural failure at a ceramic-metal interface comprising the steps of:
(a) providing a green sheet having a surface on which an input-output pad is to be disposed;
(b) screening a first screening material on the green sheet in the areas corresponding to where the input-output pad is to be disposed;
(c) line embedding a pattern on the green sheet in those areas corresponding to where the input-output pad is to be disposed;
(d) screening a second screening material on the green sheet where the pattern has been embedded;
(e) disposing said input-output pad over the patterned portion of the surface of the green sheet and into the screening material; and
(f) heat treating the green sheet, first and second screening materials and the input-output pad to create a bond between the input-output pad and the screening material surface.
In a preferred embodiment, the first and second screening materials can be the same, or different concentrations of the same material. The pattern selected is preferably an array of three dimensional shapes such as squares, rectangles, pyramids, inverted cones, dots, holes, semicircles, circles or grooves. Preferably, the pattern is extended to a depth of at least 1 mil below the surface of the green sheet.
In yet another aspect, the present invention provides a method of manufacturing a multilayer ceramic board having increased resistance to structural failure at a metal-ceramic interface comprising:
(a) providing a first green sheet having a surface on which an input-output pad is to be disposed;
(b) providing a second green sheet having a surface on which a shaped pattern is prepunched in each of those areas corresponding to where an input-output pad is to be disposed;
(c) laminating the second green sheet onto the top surface of the first green sheet;
(d) screening a material over the top surface of the second green sheet where the pattern has been punched;
(e) screening an input-output pad over the pattern portion of the second green sheet and into the screening material;
(f) heat treating the first and second green sheet, screening material and input-output pad to create a bond between the input-output pad, first and second green sheet and screening material.
In the preferred embodiment, the thickness of the second green sheet is less than 3 mils and the shaped pattern is a series of grooves, semicircles or holes.
In another preferred embodiment, a second screening material is used in step (e) to screen the input-output pad.
In a still further aspect, the present invention provides a multilayer ceramic board in a presintered state having increased strength at each ceramic-metal interface comprising:
(a) a green sheet having a surface on which an input-output pad is to be disposed;
(b) a roughened portion of the surface, the roughened portion corresponding to areas on the green sheet where an input-output pad is to be disposed; and
(c) an input-output pad screened with a screening material over the roughened portion of the surface.
In the preferred embodiment, a patterned surface is imprinted on the green sheet to create the roughened portion of the green sheet surface in (b). The pattern is preferably an array of three-dimensional shapes, such as squares, rectangles, pyramids, inverted cones, dots, holes, semicircles, circles and grooves. The pattern should extend below the surface of the green sheet to a depth at least twice as large as the average diameter of each pattern shape.
In another preferred embodiment, the roughened portion of the green sheet surface in (b) comprises:
(i) a pattern line embedded in those areas corresponding to where each input-output pad will be disposed;
(ii) a second screening material, different from that of the green sheet, screened over those areas of the green sheet corresponding to where each input-output pad will be disposed.
Preferably, the depth of the pattern in (i) extends to a depth of at least 1 mils below the surface of the green sheet and the pattern is ring shaped.
In another embodiment of the present invention the roughened portion of the surface in (b) comprises a second green sheet laminated to the first green sheet, the second green sheet having a surface on which a shaped pattern is prepunched in each of those areas corresponding to where an input-output pad is to be disposed. The pattern on the second green sheet is preferably screened with one screening material and the input-output pad is screened with another screening material. It is preferred that the thickness of the second green sheet be less than 8 mils.